Pedestrian collision warning system

ABSTRACT

A method is provided for preventing a collision between a motor vehicle and a pedestrian. The method uses a camera and a processor mountable in the motor vehicle. A candidate image is detected. Based on a change of scale of the candidate image, it may be determined that the motor vehicle and the pedestrian are expected to collide, thereby producing a potential collision warning. Further information from the image frames may be used to validate the potential collision warning. The validation may include an analysis of the optical flow of the candidate image, that lane markings prediction of a straight road, a calculation of the lateral motion of the pedestrian, if the pedestrian is crossing a lane mark or curb and/or if the vehicle is changing lanes.

BACKGROUND

1. Technical Field

The present invention relates to a driver assistance system whichprovides a collision warning with respect to a pedestrian.

2. Description of Related Art

During the last few years camera based driver assistance systems (DAS)have been entering the market; including lane departure warning (LDW),Automatic High-beam Control (AHC), pedestrian recognition, and forwardcollision warning (FCW).

Lane departure warning (LDW) systems are designed to give a warning inthe case of unintentional lane departure. The warning is given when thevehicle crosses or is about to cross the lane marker. Driver intentionis determined based on use of turn signals, change in steering wheelangle, vehicle speed and brake activation.

In image processing, the Moravec corner detection algorithm is probablyone of the earliest corner detection algorithms and defines a corner tobe a point with low self-similarity. The Moravec algorithm tests eachpixel in the image to see if a corner is present, by considering howsimilar a patch centered on the pixel is to nearby largely overlappingpatches. The similarity is measured by taking the sum of squareddifferences squared differences (SSD) between the two patches. A lowernumber indicates more similarity. An alternative approach to cornerdetection in images is based on a method proposed by Harris andStephens, which is an improvement of the method by Moravec. Harris andStephens improved upon Moravec's corner detector by considering thedifferential of the corner score with respect to direction directly,instead of using nearby patches of Moravec.

In computer vision, a widely used differential method for optical flowestimation was developed by Bruce D. Lucas and Takeo Kanade. TheLucas-Kanade method assumes that the flow is essentially constant in alocal neighborhood of the pixel under consideration, and solves thebasic optical flow equations for all the pixels in that neighborhood, bythe least squares criterion. By combining information from severalnearby pixels, the Lucas-Kanade method can often resolve the inherentambiguity of the optical flow equation. It is also less sensitive toimage noise than point-wise methods. On the other hand, since it is apurely local method, it cannot provide flow information in the interiorof uniform regions of the image.

BRIEF SUMMARY

Various methods are provided for signaling a forward collision warningusing a camera and a processor mountable in a motor vehicle.

According to a feature of the present invention, a method is providedfor preventing a collision between a motor vehicle and a pedestrianusing a camera and a processor mountable in a motor vehicle. Theprocessor may be operably connectable to the camera and the processormay be configured to capture multiple image frames from the camera ofthe environment of the motor vehicle in the field of view of the camera.The method detects an image patch in at least one of the image frames.The image patch may include a candidate image of a pedestrian in thefield of view of the camera. Detection of the candidate image may bevalidated by analyzing optical flow between the image frames of theimage patch. A collision warning or a collision prevention controlsignal may be provided when the analyzed optical flow is consistent withan upright pedestrian. Optical flow may be tracked between the imageframes of multiple image points of the image patch and the image pointsare fit to a model. The model may be a road surface model or a verticalsurface model. Based on the best fit of the image points to the model, atime-to-collision (TTC) may be determined if a collision is expected.The candidate image may be verified as an image of the uprightpedestrian and not an object in the road surface when the best fit modelis the vertical surface model.

According to features of the present invention, a method is provided forpreventing a collision between a motor vehicle and a pedestrian. Themethod uses a camera and a processor mountable in the motor vehicle. Theprocessor may be operably connectable to the camera and the processormay be configured to capture image frames from the camera of theenvironment of the motor vehicle in the field of view of the camera. Acandidate image is detected which may include the pedestrian in thefield of view of the camera. Based on a change of scale of the candidateimage, it may be determined that the motor vehicle and the pedestrianare expected to collide, thereby producing a potential collisionwarning. Further information from the image frames may be used tovalidate the potential collision warning. The validation is directed toeliminate or reduce false collision warnings with pedestrians. Acollision warning signal or a collision control signal may be provided(or inhibited) based upon the validation (or non-validation) of thepotential collision warning. The validation may include an analysis ofthe optical flow of the candidate image, that lane markings predictionof a straight road, a calculation of the lateral motion of thepedestrian, if the pedestrian is crossing a lane mark or curb and/or ifthe vehicle is changing lanes.

The validation may include predicting at least two likely paths of thevehicle, computing a region of intersection of the likely paths,determining whether the pedestrian is inside or moving towards saidregion of intersection; and the collision warning signal or thecollision control signal may be provided when the pedestrian is insideor moving towards the region of intersection. The likely paths mayinclude: a path of the present lane of travel of the vehicle asindicated by lane markings or curb, a path of travel to follow apreceding vehicle, a path which is straight ahead and a path whichavoids the vehicle going up on a curb.

The validation may also include selection of an image patch in an imageframe corresponding to where the motor vehicle will be, in apredetermined time interval; the image patch may be monitored and if thepedestrian is imaged in the image patch, then the collision warningsignal or the collision prevention control signal may be provided. Thedetermination that the motor vehicle and the pedestrian are expected tocollide may include calculating a first time-to-collision based on thechange of scale. A second time-to-collision based on the furtherinformation from the image frames may be calculated and the first andsecond times to collision compared. The collision warning signal or thecollision control signal provided, may be performed when the absolutevalue difference between the first and second time-to-collision is lessthan a threshold.

According to features of the present invention a system operable toprevent a collision between a motor vehicle and a pedestrian isprovided. The system uses a camera and a processor mountable in themotor vehicle. The processor may be operably connectable to the cameraand the processor and may be configured to capture image frames from thecamera of the environment of the motor vehicle in the field of view ofthe camera. The system may be operable to detect a candidate image. Thecandidate image may include the pedestrian in the field of view of thecamera. Based on a change of scale of the candidate image, the systemmay be operable to determine that the motor vehicle and the pedestrianare expected to collide, thereby producing a potential collisionwarning. The system may be operable to use further information from theimage frames to give a validation of the potential collision warning andprovide a collision warning signal or a collision control signal uponthe validation. The validation may include analyzing optical flow of thecandidate image, determining that lane markings predict a straight road,calculating lateral motion of the pedestrian, determining if thepedestrian is crossing a lane mark or curb or determining if the vehicleis changing lanes.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIGS. 1a and 1b show schematically two images captured from a forwardlooking camera mounted inside a vehicle as the vehicle approaches ametal fence, according to a feature of the present invention.

FIG. 2a shows a method for providing a forward collision warning using acamera mounted in host vehicle, according to a feature of the presentinvention.

FIG. 2b shows further details of determining time to collision stepshown in FIG. 2a , according to a feature of the present invention.

FIG. 3a shows an image frame of an upright surface (the back of a van),according to a feature of the present invention.

FIG. 3c shows a rectangular region which is primarily of a road surface,according to a feature of the present invention.

FIG. 3b shows the vertical motion of points δy as a function of verticalimage position (y) for FIG. 3a , according to an feature of the presentinvention.

FIG. 3d shows the vertical motion of points δy as a function of verticalimage position (y) for FIG. 3c , according to an feature of the presentinvention.

FIG. 4a shows an image frame which includes an image of a metal fencewith horizontal lines and a rectangular patch, according to a feature ofthe present invention.

FIGS. 4b and 4c show more details of the rectangular patch shown in FIG.4a , according to a feature of the present invention.

FIG. 4d illustrates a graph of vertical motion of points (δy) versusvertical point position (y), according to a feature of the presentinvention.

FIG. 5 illustrates another example of looming in an image frame,according to a feature of the present invention.

FIG. 6 shows a method for providing a forward collision warning trap,according to a feature of the present invention.

FIGS. 6a and 6b show examples of two warning zones respectivelyaccording to an exemplary feature of the present invention.

FIG. 6c shows a method, according to a feature of the present invention.

FIG. 6d shows further details of the validation step shown in FIG. 6c ,according to a feature of the present invention.

FIG. 6e shows a method, according to a feature of the present invention.

FIG. 6f shows further details of the validation step shown in FIG. 6e ,according to a feature of the present invention.

FIGS. 7a and 7b show examples of a forward collision trap warning beingtriggered on walls, according to an exemplary feature of the presentinvention.

FIG. 7c shows an example of a forward collision trap warning beingtriggered on boxes, according to an exemplary feature of the presentinvention.

FIG. 7d shows an example of a forward collision trap warning beingtriggered on sides of a car, according to an exemplary feature of thepresent invention.

FIG. 8a shows an example of objects with strong vertical lines on a box,according to an aspect of the present invention.

FIG. 8b shows an example of objects with strong vertical lines on a lamppost, according to an aspect of the present invention.

FIGS. 9 and 10 illustrate a system which includes a camera or imagesensor mounted in a vehicle, according to an aspect of the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The features are described below to explain the presentinvention by referring to the figures.

Before explaining features of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other features or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

By way of introduction, embodiments of the present invention aredirected to a forward collision (FCW) system. According to U.S. Pat. No.7,113,867, a image of lead vehicle is recognized. The width of thevehicle may be used to detect a change in scale or relative scale Sbetween image frames and the relative scale is used for determining timeto contact. Specifically, for example width of the lead vehicle, have alength (as measured for example in pixels or millimeters) in the firstand second images represented by w(t1) and w(t2) respectively. Then,optionally the relative scale is S(t)=w(t2)/w(t1).

According to the teachings of U.S. Pat. No. 7,113,867, the forwardcollision (FCW) system depends upon recognition of an image of anobstruction or object, e.g. lead vehicle for instance, as recognized inthe image frames. In the forward collision warning system, as disclosedin U.S. Pat. No. 7,113,867, the scale change of a dimension, e.g width,of the detected object e.g. vehicle is used to compute time-to-contact(TTC). However, the object is first detected and segmented from thesurrounding scene. The present application describes a system in whichoptical flow is used to determine the time to collision TTC and/orlikelihood of collision and issue an FCW warning if required. Opticalflow causes the looming phenomenon in perception of images which appearlarger as objects being imaged get closer. Object detection and/orrecognition may be performed or object detection and/or recognition maybe avoided, according to different features of the present invention.

The looming phenomenon has been widely studied in biological systems.Looming appears to be a very low level visual attention mechanism inhumans and can trigger instinctive reactions. There have been variousattempts in computer vision to detect looming and there was even asilicon sensor design for detection of looming in the pure translationcase.

Looming detection may be performed in real world environments withchanging lighting conditions, complex scenes including multiple objectsand host vehicle which includes both translation and rotation.

The term “relative scale” or “change of scale” as used herein refers tothe relative size increase (or decrease) of an image patch in an imageframe and a corresponding image patch in a subsequent image frame.

Reference is now made to FIGS. 9 and 10 which illustrate a system 16including a camera or image sensor 12 mounted in a vehicle 18, accordingto an aspect of the present invention. Image sensor 12, imaging a fieldof view in the forward direction delivers images in real time and theimages are captured in a time series of image frames 15. An imageprocessor 14 may be used to process image frames 15 simultaneouslyand/or in parallel to serve a number of driver assistance systems. Thedriver assistance systems may be implemented using specific hardwarecircuitry with on board software and/or software control algorithms instorage 13. Image sensor 12 may be monochrome or black-white, i.e.without color separation or image sensor 12 may be color sensitive. Byway of example in FIG. 10, image frames 15 are used to serve pedestrianwarning (PW) 20, lane departure warning (LDW) 21, forward collisionwarning (FCW) 22 based on target detection and tracking according to theteachings of U.S. Pat. No. 7,113,867, forward collision warning based onimage looming (FCWL) 209 and/or forward collision warning 601 based onan FCW trap (FCWT) 601. Image processor 14 is used to process imageframes 15 to detect looming in an image in the forward field of view ofcamera 12 for forward collision warning 209 based on image looming andFCWT 601. Forward collision warning 209 based on image looming andforward collision warning based on traps (FCWT) 601 may be performed inparallel with conventional FCW 22 and with the other driver assistancefunctions, pedestrian detection (PW) 20, lane departure warning (LDW)21, traffic sign detection, and ego motion detection. FCWT 601 may beused to validate the conventional signal from FCW 22. The term “FCWsignal” as used herein refers to a forward collision warning signal. Theterms “FCW signal”, “forward collision warning”, and “warning” are usedherein interchangeably.

A feature of the present invention is illustrated in FIGS. 1a and 1bwhich show an example of optical flow or looming. Two images capturedare shown from a forward looking camera 12 mounted inside a vehicle 18as vehicle 18 approaches a metal fence 30. The image in FIG. 1a shows afield and a fence 30. The image in FIG. 1b shows the same features withvehicle 18 closer if a small rectangular p 32 of the fence (marked indotted line) is viewed it may be possible see that the horizontal lines34 appear to spread out as vehicle 18 approaches fence 30 in FIG. 1 b.

Reference is now made to FIG. 2a which shows a method 201 for providinga forward collision warning 209 (FCWL 209) using camera 12 mounted inhost vehicle 18, according to a feature of the present invention. Method201 does not depend on object recognition of an object in the forwardview of vehicle 18. In step 203 multiple image frames 15 are acquired bycamera 12. The time interval between capture of image frames is zit. Apatch 32 in image frame 15 is selected in step 205 and a relative scale(S) of patch 32 is determined in step 207. In step 209, thetime-to-collision (TTC) is determined based on the relative scale (S)and the time interval (Δt) between frames 15.

Reference is now made to FIG. 2b which shows further details ofdetermining time to collision step 209 shown in FIG. 2a , according to afeature of the present invention. In step 211 multiple image points in apatch 32 may be tracked between image frames 15. In step 213 the imagepoints of may be fit to one or more models. A first model may be avertical surface model which may include objects such as a pedestrian, avehicle, a wall, bushes, trees or a lamp post. A second model may be aroad surface model which considers features of image points on the roadsurface. A mixed model may include one or more image points from theroad and one or more image points from an upright object For modelswhich assume at least a portion of the image points of an upright objectmultiple time-to-collision (TTC) values may be computed. In step 215,the best fit of the image points to a road surface model, a verticalsurface model, or a mixed model enables selection of thetime-to-collision (TTC) value. A warning may be issued based on thetime-to-collision (TTC) being less than a threshold and when the bestfit model is the vertical surface model or a mixed model.

Alternatively, step 213 may also include in the image frames 15, thedetection of a candidate image. The candidate image may be a pedestrianor a vertical line of a vertical object such as lamppost for example. Ineither case of a pedestrian or a vertical line, patch 32 may be selectedso as to include the candidate image. Once patch 32 has been selected itmay then be possible to perform a verification that the candidate imageis an image of an upright pedestrian and/or a vertical line. Theverification may confirm that the candidate image is not an object inthe road surface when the best fit model is the vertical surface model.

Referring back to FIGS. 1a and 1b , sub-pixel alignment of patch 32 fromthe first image shown in FIG. 1a to the second image shown in FIG. 1bmay produce a size increase or increase in relative scale S by 8%(S=1.08) (step 207). Given the time difference between the images ofΔt=0.5 sec, the time to contact (TTC) can be computed (step 209) usingequation 1 below:

$\begin{matrix}{{TTC} = {\frac{\Delta \; t}{S - 1} = {0.5008 = {6.25\; \sec}}}} & (1)\end{matrix}$

If vehicle 18 speed v in known (v=4.8 m/s), the distance Z to the targetcan also be computed using equation 2 below:

$\begin{matrix}{Z = {\frac{\upsilon*\Delta \; t}{S - 1} = {\frac{4.8*0.5}{1.08 - 1} = {30\; m}}}} & (2)\end{matrix}$

FIGS. 3b and 3d show the vertical motion of points δy as a function ofvertical image position (y), according to an feature of the presentinvention. Vertical motion δy is zero at the horizon and negative valuesare below the horizon. Vertical motion of points δy is shown in equation3 below.

$\begin{matrix}{{\delta \; y} = \frac{\Delta \; {Z\left( {y - y_{0}} \right)}}{Z}} & (3)\end{matrix}$

Equation (3) is a linear model relating y and δy and has effectively twovariables. Two points may be used to solve for the two variables.

For vertical surfaces the motion is zero at the horizon (y₀) and changeslinearly with image position since all the points are at equal distanceas in the graph shown in FIG. 3b . For road surfaces, points lower inthe image are closer (Z is smaller) as shown in equation 4 below:

$\begin{matrix}{Z = \frac{f\; H}{y - y_{0}}} & (4)\end{matrix}$

and so the image motion (δy) increases at more than linear rate as shownin equation 5 below and in the graph of FIG. 3d .

$\begin{matrix}{{\delta \; y} = \frac{\Delta \; {Z\left( {y - {y\; 0}} \right)}^{2}}{f\; H}} & (5)\end{matrix}$

Equation (5) is a restricted second order equation with effectively twovariables.

Again, two points may be used to solve for the two variables.

Reference is now made to FIGS. 3a and 3c which represent different imageframes 15. In FIGS. 3a and 3c two rectangular regions are shown bydotted lines. FIG. 3a shows an upright surface (the back of a van). Thesquare points are points that were tracked (step 211) and the motionmatches (step 213) the motion model for an upright surface shown in thegraph of image motion (δy) versus point height y in FIG. 3b . The motionof the triangular points in FIG. 3a do not match the motion model for anupright surface. Reference is now made to FIG. 3c which shows arectangular region primarily of a road surface. The square points arepoints that match a road surface model shown in the graph of imagemotion (δy) versus point height y in FIG. 3d . The motion of triangularpoints do not match the motion model for the road surface and areoutliers. The task in general therefore is to determine which pointsbelong to the model (and to which model) and which points are outlierswhich may be performed by a robust fit approach as explained below.

Reference is now made to FIGS. 4a, 4b, 4c and 4d which show a typicalscenario of a mixture of two motion models found in an image, accordingto an feature of the present invention. FIG. 4a shows an image frame 15including an image of a metal fence 30 with horizontal lines 34 andrectangular patch 32 a. Further detail of patch 32 a are shown in FIGS.4b and 4c . FIG. 4b shows detail of patch 32 a of a previous image frame15 and FIG. 4c shows detail 32 a in a subsequent image frame 15 whenvehicle 18 is closer to fence 30. Image points are shown as squares,triangles and circles in FIGS. 4c and 4d on the upright obstacle 30 andimage points are shown on the road surface leading up to the obstacle30. Tracking points inside the rectangular region 32 a show that somepoints in the lower part of region 32 a correspond to a road model andsome points in the upper part of region 32 a correspond to an uprightsurface model. FIG. 4d illustrates a graph of vertical motion of points(δy) versus vertical point position (y). The recovered model showngraphically in FIG. 4d has two parts a curved (parabolic) section 38 aand a linear section 38 b. The transition point between sections 38 aand 38 b corresponds to the bottom of upright surface 30. The transitionpoint is also marked by a horizontal dotted line 36 shown in FIG. 4c .There are some points shown by triangles in the FIGS. 4b and 4c thatwere tracked but did not match the model, some tracked points which didmatch the model are shown by squares and some points that did not trackwell are shown as circles.

Reference is now made to FIG. 5 which illustrates another example oflooming in an image frame 15. In image frame 15 of FIG. 5, there is noupright surface in patch 32 b, only clear road ahead and the transitionpoint between the two models is at the horizon marked by dotted line 50.

Estimation of the Motion Model and Time to Collision (TTC)

The estimation of the motion model and time to contact (TTC) (step 215)assumes we are provided a region 32, e.g. a rectangular region in imageframe 15. Examples of rectangular regions are rectangles 32 a and 32 bshown in FIGS. 3 and 5 for example. These rectangles may be selectedbased on detected objects such as pedestrians or based on the hostvehicle 18 motion.

1. Tracking Points (Step 211):

(a) A rectangular region 32 may be tessellated into 5×20 grid ofsub-rectangles.

(b) For each sub-rectangle, an algorithm may be performed to find acorner of an image, for instance by using the method of Harris andStephens and this point may be tracked. Using the best 5×5 Harris Pointthe eigenvalues of the matrix below may be considered,

$\begin{matrix}\begin{pmatrix}{\sum\; {\delta \; x^{2}}} & {\sum\; {\delta \; x\; \delta \; y}} \\{\sum\; {\delta \; x\; \delta \; y}} & {\sum\; {\delta \; y^{2}}}\end{pmatrix} & (6)\end{matrix}$

and we look for two strong eigenvalues.

(c) Tracking may be performed by exhaustive search for the best some ofsquared differences (SSD) match in a rectangular search region of widthW and height H. The exhaustive search at the start is important since itmeans that a prior motion is not introduced and the measurements fromall the sub-rectangles are more statistically independent. The search isfollowed by fine tuning using an optical flow estimation using forinstance the method Lukas Kanade. The Lukas Kanade method allows forsub-pixel motion.

2. Robust Model Fitting (Step 213):

(a) Pick two or three points randomly from the 100 tracked points.

(b) The number of pairs (N_(pairs)) picked depends the vehicle speed (v)and is given for instance by:

N _(pairs)=min(40,max(5,50−v))  (7)

where v is in meter/second. The number of triplets (N_(triplets)) isgiven by:

N _(triplets)=50−N _(pairs)  (8)

(c) For two points, two models may be fit (step 213). One model assumesthe points are on an upright object. The second model assumes they areboth on the road.

(d) For three points two models may also be fit. One model assumes thetop two points are on an upright object and the third (lowest) point ison the road. The second model assumes the upper point is on an uprightobject and the lower two are on the road.

Two models may be solved for three points by using two points to solvefor the first model (equation 3) and then using the resulting y₀ and thethird point to solve for the second model (equation 5).

(e) Each model in (d) gives a time-to-collision TTC value (step 215).Each model also gets a score based on how well the 98 other points fitthe model. The score is given by the Sum of the Clipped Square of theDistance (SCSD) between the y motion of the point and predicted modelmotion. The SCSD value is converted into a probability like function:

where is the number of points (N=98).

$\begin{matrix}{{score} = {\exp \frac{- {SCSD}}{N}}} & (9)\end{matrix}$

(f) Based on the TTC value, vehicle 18 speed and assuming the points areon stationary objects, the distance to the points: Z=v×TTC may becomputed. From the x image coordinate of each image point distance, thelateral position in world coordinates may be computed:

$\begin{matrix}{X = \frac{x\; Z}{f}} & (10) \\{{\Delta \; X} = \frac{\delta \; x\; Z}{f}} & (11)\end{matrix}$

(g) The lateral position at time TTC is computed thus. A binary LateralScore requires that at least one of the points from the pair or tripletmust be in the vehicle 18 path.

3. Multiframe Scores: At each frame 15 new models may be generated, eachwith its associated TTC and score. The 200 best (highest scoring) modelsmay be kept from the past 4 frames 15 where the scores are weighted:

where n=0.3 is the age of the score and α=0:95.

score(n)=α^(n)score  (12)

4. FCW Decision: the actual FCW warning is given if any of the followingthree conditions occurs:

(a) The TTC for the model with the highest score is below the TTCthreshold and the score is greater than 0.75 and

$\begin{matrix}{\frac{\sum_{{TTC} < {Threshold}}{scores}}{\sum_{allmodels}{scores}} > 0.3} & (13)\end{matrix}$

(b) The TTC for the model with the highest score is below the TTCthreshold and

$\begin{matrix}{\frac{\sum_{{TTC} < {Threshold}}{scores}}{\sum_{allmodels}{scores}} > 0.5} & (14)\end{matrix}$

(c)

$\begin{matrix}{\frac{\sum_{{TTC} < {Threshold}}{scores}}{\sum_{allmodels}{scores}} > 0.75} & (15)\end{matrix}$

FIGS. 3 and 4 have shown how to robustly provide a FCW warning forpoints inside a given rectangle 32. How the rectangle is defined dependson the application as shown by other exemplary features of FIGS. 7a-7dand 8a , 8 b.

FCW Trap for General Stationary Objects

Reference is now made to FIG. 6 which shows a method 601 for providing aforward collision warning trap (FCWT) 601, according to a feature of thepresent invention. In step 203 multiple image frames 15 are acquired bycamera 12. In step 605, patch 32 is selected in an image frame 15 whichcorresponds to where motor vehicle 18 will be in a predetermined timeinterval. Patch 32 is then monitored in step 607. In decision step 609if a general object is imaged and detected in patch 32, a forwardcollision warning is issued in step 611. Otherwise capturing of imagesframes continues with step 203.

FIGS. 7a and 7b show examples of the FCWT 601 warning being triggered onwalls 70, in FIG. 7d sides of a car 72 and in FIG. 7c on boxes 74 a and74 b, according to an exemplary feature of the present invention. FIGS.7a-7d are examples of general stationary objects which require no priorclass based detection. The dotted rectangular region is defined as atarget W=1 m wide at a distance where the host vehicle will be in t=4seconds.

Where v is the vehicle 18 speed, H is the height of camera 12 and w andy are a

$\begin{matrix}{Z = {vt}} & (16) \\{w = \frac{fW}{Z}} & (17) \\{y = \frac{fH}{Z}} & (18)\end{matrix}$

rectangle width and vertical position in the image respectively. Therectangular region is an example of an FCW trap. If an object “falls”into this rectangular region, the FCW Trap may generate a warning if theTTC is less than a Threshold.

Improving Performance Using Multiple Traps:

In order to increase the detection rate, the FCW trap may be replicatedinto 5 regions with 50% overlap creating a total trap zone of 3 m width.

Dynamic position of the FCW trap may be selected (step 605) on yaw rate:the trap region 32 may be shifted laterally based on the vehicle 18 pathdetermined from a yaw rate sensor, the vehicle 18 speed and dynamicalmodel of the host vehicle 18.

The FCW trap 601 concept can be extended to objects consisting mainly ofvertical (or horizontal lines). A possible problem with using the pointbased techniques on such objects is that the good Harris (corner) pointsare most often created by the vertical lines on the edge of the objectintersecting horizontal lines on the distant background. The verticalmotion of these points will be like the road surface in the distance.

FIGS. 8a and 8b show examples of objects with strong vertical lines 82on a lamp post 80 in FIG. 8b and on a box 84 in FIG. 8a . Vertical lines82 are detected in the trap zone 32. The detected lines 82 may betracked between images. Robust estimation may be performed by pairing uplines 82 from frame to frame and computing a TTC model for each linepair, assuming a vertical object, and then giving a score based on theSCSD of the other lines 82. Since the number of lines may be small,often all combinatorially possibly line pairs are tested. Only linepairs where there is significant overlap are used. In the case ofhorizontal lines, triplets of lines are also giving two models as withpoints.

FCW Trap for Validating Collision Warning Signals with Pedestrians

Special classes of objects such as vehicles and pedestrians can bedetected in image 15 using pattern recognition techniques. According tothe teachings of U.S. Pat. No. 7,113,867, these objects are then trackedover time and an FCW 22 signal can be generated using the change inscale. However, before giving a warning it is important to validate theFCW 22 signal using an independent technique. Validating the FCW 22signal using an independent technique, for instance using method 209(FIG. 2b ) may be particularly important if system 16 will activate thebrakes. In Radar/vision fusion systems the independent validation cancome from the radar. In a vision only based system 16, the independentvalidation comes from an independent vision algorithm.

Object (e.g. pedestrian, lead vehicle) detection is not the issue. Veryhigh detection rate can be achieved with a very low false rate. Afeature of the present invention is to generate a reliable FCW signalwithout too many false alarms that will irritate the driver, or worse,cause the driver to brake unnecessarily. A possible problem withconventional pedestrian FCW systems is to avoid false forward collisionwarnings as the number of pedestrians in the scene is large but thenumber of true forward collision situations is very small. Even a 5%false rate would mean the driver would get frequent false alarms andprobably never experience a true warning.

Pedestrian targets are particularly challenging for FCW systems becausethe targets are non-rigid making tracking (according to the teachings ofU.S. Pat. No. 7,113,867) difficult and scale change in particular isvery noisy. Thus the robust model (method 209) may be used to validatethe forward collision warning on pedestrians. The rectangular zone 32may be determined by a pedestrian detection system 20. A FCW signal maybe generated only if target tracking performed by FCW 22, according toU.S. Pat. No. 7,113,867 and the robust FCW (method 209) give a ITCsmaller than one or more threshold values which may or may not bepreviously determined. Forward collision warning FCW 22, may have adifferent threshold value from the threshold used in the robust model(method 209).

One of the factors that can add to the number of false warning is thatpedestrians typically appear in less structured roads where the driversdriving pattern can be quite erratic including sharp turns and lanechanges. Thus some further constraints may need to be included onissuing a warning:

When a curb or lane mark is detected the FCW signal is inhibited if thepedestrian is on the far side of the curb or lane and neither of thefollowing conditions occur:

1. The pedestrian is crossing the lane mark or curb (or approaching veryfast). For this it may be important to detect the pedestrian's feet.

2. The host vehicle 18 is not crossing the lane mark or curb (asdetected by an LDW 21 system for example).

The drivers intentions are difficult to predict. If the driver isdriving straight, has not activated turn signals and there are no lanemarkings predicting otherwise it is reasonable to assume that the driverwill continue straight ahead. Thus, if there is a pedestrian in path andTTC is below threshold an FCW signal can be given. However if the driveris in a turn it is equally likely that he/she will continue in the turnor break out of the turn and straighten out. Thus, when yaw rate isdetected, an FCW signal may only be given if the pedestrian is in pathassuming the vehicle 18 will continue at the same yaw and also thepedestrian is in path if the vehicle straightens out.

According to a feature of the present invention, likely paths of vehicle18 are predicted. The likely paths may include proceeding straightahead, continue on a curve, following a lane marking or curb, avoidinggoing up on a curb and/or following a preceding vehicle.

In order to avoid false positive collision warnings with pedestrians,FCW signal may be inhibited if there is a likely path for the motorvehicle which does not include a pedestrian with the TTC to thepedestrian less than the threshold.

Pedestrian Lateral Motion

The pedestrian typically moves slowly compared to the host vehicle 18and therefore the longitudinal motion of the pedestrian can be ignored.The lateral motion of the pedestrian, whether into the host vehicle 18path or away from the host vehicle 18 path is critical.

As the longitudinal distance to the pedestrian decreases there will beoutward image motion (optical flow):

${\delta \; x_{of}} = \frac{v\; \Delta \; {T\left( {x - x_{0}} \right)}}{Z}$

where v is vehicle 18 speed, vΔT=ΔZ, is the vehicle 18 longitudinalmotion and x₀ is the x coordinate of the focus of expansion (FOE):

$x_{0} \approx \frac{w_{y}\Delta \; T}{2}$

where w_(y) is the yaw rate. This is a simplified motion model thatassumes no lateral slippage of the host vehicle 18.

So the lateral motion from a tracked point on the pedestrian can becomputed from the lateral image motion δx:

${\Delta \; X} = \frac{\left( {{\delta \; x_{of}} - x_{0}} \right)Z}{f\; \Delta \; T}$

The current lateral position of the pedestrian (X_(T=0)) or simply X isgiven by:

$X_{T = 0} = \frac{xZ}{f}$

The lateral position of the pedestrian at T=TTC is given by:

X _(T=TTC) =X+ΔX*TTC

Estimating Collision Path with a Pedestrian

To determine whether the host vehicle 18 is on a collision course withthe pedestrian and a warning should be issued, two warning zones may bedefined in world coordinate space. Warning zone 1 is shown in FIG. 6aand warning zone 2 is shown in FIG. 6 b:

-   -   1. Warning zone 1: The intersection of a rectangular region        spanning 1 m to the left and right of vehicle 18 and of length        v*TTC with a curved region 62 a spanning 1 m to the left and        right of the vehicle assuming vehicle 18 continues on a path        predicted by the yaw rate.    -   2. Warning zone 2: The intersection of a rectangular region        spanning 0.1 m to the left and right of vehicle 18 and of length        v*TTC with a curved region 62 b spanning 0.1 m to the left and        right of vehicle 18 assuming vehicle 18 continues on a path        predicted by the yaw rate. Region 62 b is further intersected        with the region to the right of the left lane mark 64 or curb if        detected and with the region to the left of the right lane mark        or curb if detected.

Triggering a Pedestrian FCW Warning

A pedestrian FCW warning may be given if the following hold:

-   -   1. A pedestrian has been detected with high confidence.    -   2. TTC₁, based on distance and vehicle speed is below a        threshold T₁:

${TTC}_{1} = {\frac{Z}{v} < T_{1}}$

-   -   3. TTC₂, based on FCW validation (method 201 for example) is        below threshold T₂ and the FCW decision is ‘true’.    -   4. The two predicted TTC's are not very different:    -   5. The current pedestrian position (X, Z) at T=0 is inside        warning zone 1.

|TTC₁−TTC₁ |<T ₃

-   -   6. The predicted pedestrian position (X_(T=TTC), Z_(T=TTC)) is        inside warning zone 2.

Reference is now made to FIG. 6c which shows a method 621, according toa feature of the present invention. Method 621 is a method forpreventing a collision between a motor vehicle 18 and a pedestrian. Instep 623, an image patch 32 as a candidate image is detected from atleast one image frames 15 captured in the field of view of camera 12.The candidate image may include an image of a pedestrian. Step 621 isvalidated in step 625 by an analysis of an optical flow between theimage frames 15 for image patch 32. In decision 627, if the optical flowbetween the image frames 15 for image patch 32 is consistent with anupright pedestrian, a collision warning signal or collision controlsignal is provided in step 629. The optical flow as discussed above maybe by virtue of the lateral motion of the pedestrian in the field ofview of camera 12. If the optical flow between the image frames 15 forimage patch 32 is not consistent with an upright pedestrian then acandidate image is detected from at least one image frames 15 capturedin the field of view of camera 12 in step 623.

Reference is now made to FIG. 6d which shows further details ofvalidation step 625 shown in FIG. 6c , according to another feature ofthe present invention. In step 633, the optical flow is tracked betweenimage frames 15 for multiple image points of image patch 32. Themultiple image points of image patch 32 are then fitted to a model instep 635. If a sufficient fit is not found in decision 637, then acollision warning or collision control signal may be inhibited. Acandidate image may be detected again from image frames 15.

If a best fit is found in decision box 637, then in decision box 639 aif a collision is expected, that is the best fit corresponds to anupright pedestrian or another vertical object, then a time-to-collisionmay be determined (step 639 b). If a collision is not expected, forinstance because the object is not upright, then a candidate image maybe detected again in step 623.

Reference is now made to FIG. 6e which shows a method 641, according toa feature of the present invention. Method 641 is a method forprevention of a collision between a motor vehicle and a pedestrian. Instep 643 a candidate image is detected in the field of view of camera 12mounted in motor vehicle 18. The candidate image may include an image ofa pedestrian. In decision block 645 it is determined if a change ofscale is detected in the candidate image. If a sufficient change ofscale is not detected in the candidate image, e.g. time-to-collision isgreater than threshold, then detection of a candidate image may continuein step 643. If a sufficient change of scale is detected in thecandidate image, and if in decision block 647 the vehicle and apedestrian are expected to collide for instance based on TTC, then thepotential collision warning is validated in step 649 according to any ofthe methods disclosed herein. Otherwise, detection of a candidate imagemay continue in step 643. Decision step 647 may include calculating afirst time-to-collision based on the change of scale. If the motorvehicle 18 and a pedestrian are expected to collide then furtherinformation 655 from image frames 15 is used for validation in step 649.Potential collision warning 651 may be validated or inhibited indecision block 653. If validated then a collision warning signal or acollision control signal is provided in step 645, otherwise detection ofa candidate image continues in step 643.

A second time-to-collision may be calculated based on information 655from image frames 15, by a comparison made between the first and secondtimes to collision. The collision warning signal or the collisioncontrol signal provided in step 645 may be performed when the absolutevalue difference between the first and second time-to-collision is lessthan a 10 threshold.

Reference is now made to FIG. 6f which shows further details ofvalidation step 649 shown in FIG. 6e , according to a feature of thepresent invention. In step 605, a patch 32 is selected in one of theimage frames 15. The selection of patch 32 is based on where motorvehicle 18 will be in a predetermined time interval followed by patchbeing monitored in step 607. Steps 605 and 607 are described in greaterdetail above in the description of method 601. In decision 609 a, if apedestrian is imaged in patch 32, then a collision warning signal or acollision control signal is provided in step 645, otherwise detection ofa candidate image continues in step 643.

Validation step 649 may include an analysis of an optical flow of thecandidate image. In the case of a pedestrian the optical flow may be thelateral motion of the pedestrian. Validation step 649 may also include adetermination that lane markings predict a straight road. The straightroad may be indicative that a pedestrian may be more likely to cross theroad as opposed to the curved road giving the impression that apedestrian is already in road. Further validation 649 may also include adetermination that a pedestrian is crossing a lane mark or curb.Avoidance of the provision of a collision warning signal or collisioncontrol signal (step 645) may take into consideration that motor vehicle18 is changing lanes and therefore the pedestrian will not be in thepath of motor vehicle 18.

Alternatively, a collision warning and/or collision control may beprovided (or not inhibited) without necessarily determining a change inscale in the candidate image (step 645). A candidate image of apedestrian is detected. The location of point of contact between thepedestrian and the road, e.g. a foot of the pedestrian is detected. Ifthe feet are determined to be in one or more likely vehicle paths thenthe collision warning and/or collision control may be provided or notinhibited in a validation step 649. The predicted vehicle path may beprojected onto the image and if the pedestrian's feet overlap thepredicted path then the collision warning and/or collision control maybe provided or not inhibited in a validation step 649. Alternatively,vehicle path may be predicted in world coordinates and the known feetlocation and the known camera perspective projection are used to locatethe pedestrian in world coordinates.

The term “collision control signal” as used herein may include but isnot limited to a control signal which initiates braking of the vehiclewith or without driver intervention.

The term “collision warning” as used herein is a signal, typicallyaudible, to the driver of the motor vehicle that driver intervention maybe required to prevent a collision.

The indefinite articles “a”, “an” is used herein, such as “an image”, “arectangular region” have the meaning of “one or more” that is “one ormore images” or “one or more rectangular regions”.

The term “validation” and “verification” are used hereininterchangeably.

Although selected features of the present invention have been shown anddescribed, it is to be understood the present invention is not limitedto the described features. Instead, it is to be appreciated that changesmay be made to these features without departing from the principles andspirit of the invention, the scope of which is defined by the claims andthe equivalents thereof.

What is claimed is:
 1. A computerized method using a camera mountable ina motor vehicle, the method comprising: acquiring a plurality of imageframes from the camera with a known time interval; detecting an imagepatch in at least one of the image frames, wherein the image patchincludes a candidate image of a pedestrian in the field of view of thecamera; responsive to the motor vehicle moving forward, tracking opticalflow between the image frames of an image point of the pedestrian; fromhorizontal image motion of the tracked point of the pedestrian relativeto the horizontal image coordinate of the focus of expansion, computinglateral motion of the pedestrian responsive to longitudinal distance tothe pedestrian; and estimating the lateral position of the pedestrian ata time-to-collision from the lateral motion of the pedestrian.
 2. Themethod of claim 1, further comprising determining a firsttime-to-collision based on the longitudinal distance to the pedestrianand a speed of the motor vehicle and comparing the firsttime-to-collision to a threshold; and determining a secondtime-to-collision based on optical flow of the image patch and comparingthe second time-to-collision to a threshold.
 3. The method of claim 2,further comprising when the first and second times to collision are ofsimilar value and both less than the respective thresholds, providing awarning or issuing a control signal; or inhibiting a warning orinhibiting a control signal if the motor vehicle when the first andsecond times to collision are not of similar value and not both lessthan the respective thresholds.
 4. The method of claim 2, furthercomprising: for a plurality of models selected from the group consistingof: (i) a road surface model wherein a portion of the image points ismodeled to be imaged from a road surface (ii) vertical surface modelwherein a portion of the image points is modeled to be imaged from asubstantially vertical object and (iii) a mixed model wherein a firstportion of the image points is modeled to be imaged from a road surfaceand a second portion of the image points is modeled to be imaged from asubstantially vertical object, fitting the tracked optical flow of atleast a portion of the image points to produce a plurality of fits tothe models; using at least a portion of the image points, scoring thefits of the tracked optical flow to the respective models to producerespective scores; and determining if a collision is expected and thesecond time-to-collision by selecting the model with a score whichcorresponds to a best fit of the image points to the tracked opticalflow.
 5. The method of claim 4, further comprising: determining that acollision is expected based on the best fit of the image points to thetracked optical flow corresponding to the vertical surface model or themixed model.
 6. The method of claim 1, further comprising providing acollision warning or a collision prevention control signal if thepedestrian is on the far side of a curb or a lane mark and is crossingthe curb or the lane mark; or inhibiting a collision warning or acollision prevention control signal if the pedestrian is on the far sideof a curb or a lane mark and the pedestrian is not crossing the curb orthe lane mark.
 7. The method of claim 1, further comprising: predictingat least two most likely paths of the host vehicle; computing a regionof intersection of said at least two paths; determining whether thepedestrian is inside or moving towards said region of intersection; andproviding a collision warning signal or a collision control signal whenthe pedestrian is inside or moving towards said region of intersection;or inhibiting a collision warning signal or a collision control signalwhen the pedestrian is not inside and not moving towards said region ofintersection.
 8. The method of claim 1, further comprising: inputting orcomputing from the image frames a yaw rate of the motor vehicle; andestimating from the yaw rate the horizontal image coordinate of thefocus of expansion.
 9. A system comprising: a camera mountable in amotor vehicle; and a processor mountable in the motor vehicle, whereinthe processor is operably connectable to the camera; wherein theprocessor is configured to: acquire a plurality of image frames from thecamera with a known time interval; detect an image patch in at least oneof the image frames, wherein the image patch includes a candidate imageof a pedestrian in the field of view of the camera; responsive to theforward motion of the motor vehicle, track optical flow between theimage frames of an image point of the pedestrian; from horizontal imagemotion of the tracked point of the pedestrian relative to the horizontalimage coordinate of the focus of expansion, compute lateral motion ofthe pedestrian responsive to longitudinal distance to the pedestrian;and estimate the lateral position of the pedestrian at atime-to-collision from the lateral motion of the pedestrian.
 10. Thesystem according to claim 9, wherein the processor is further configuredto: determine a first time-to-collision based on the longitudinaldistance to the pedestrian and a speed of the motor vehicle and comparethe first time-to-collision to a threshold; and determine a secondtime-to-collision based on optical flow of the image patch and comparingthe second time-to-collision to a threshold.
 11. The system of claim 10,wherein the processor is further configured to: provide a warning orissue a control signal when the first and second times to collision areof similar value and both less than the respective thresholds; or whenthe first and second times to collision are not of similar value and notboth less than the respective thresholds inhibit a warning or inhibit acontrol signal.
 12. The system of claim 10, wherein the processor isfurther configured to: for a plurality of models selected from the groupconsisting of: (i) a road surface model wherein a portion of the imagepoints is modeled to be imaged from a road surface (ii) vertical surfacemodel wherein a portion of the image points is modeled to be imaged froma substantially vertical object and (iii) a mixed model wherein a firstportion of the image points is modeled to be imaged from a road surfaceand a second portion of the image points is modeled to be imaged from asubstantially vertical object, fit the tracked optical flow of at leasta portion of the image points to produce a plurality of fits to themodels; using at least a portion of the image points, score the fits ofthe tracked optical flow to the respective models to produce respectivescores; and determine if a collision is expected and the secondtime-to-collision by selecting the model with a score which correspondsto a best fit of the image points to the tracked optical flow.
 13. Thesystem of claim 10, wherein the processor is further configured to:determine that a collision is expected based on the best fit of theimage points to the tracked optical flow corresponding to the verticalsurface model or the mixed model.
 14. The system of claim 9, wherein theprocessor is further configured to: provide a collision warning or acollision prevention control signal if the pedestrian is on the far sideof a curb or a lane mark and is crossing the curb or the lane mark; orinhibit a collision warning or a collision prevention control signal ifthe pedestrian is on the far side of a curb or a lane mark and thepedestrian is not crossing the curb or the lane mark.
 15. The system ofclaim 9, wherein the processor is further configured to: predict atleast two most likely paths of the host vehicle; compute a region ofintersection of said at least two paths; determine whether thepedestrian is inside or moving towards said region of intersection; andprovide a collision warning signal or a collision control signal whenthe pedestrian is inside or moving towards said region of intersection;or when the pedestrian is not inside and not moving towards said regionof intersection, inhibit a collision warning signal or a collisioncontrol signal.
 16. The system of claim 9, wherein the processor isfurther configured to: input or compute from the image frames a yaw rateof the motor vehicle; and estimate from the yaw rate the horizontalimage coordinate of the focus of expansion.